1. Field of the Invention
The invention relates generally to electrical switches and, more particularly, to high voltage, high current disconnect switches suitable for use in isolated phase bus duct.
2. Description of the Prior Art
In order to efficiently supply electrical energy to consumers, utility companies employ large generators typically having capacities of several hundred million watts. This energy can be generated most efficiently at medium voltages of, for example, 22,000 volts. However, it is stepped up by transformers to much higher voltages in order to most economically transmit the energy over long distances. The connection between the generator and the step-up power transformer is usually made by isolated phase bus duct consisting of a plurality of phase conductors each having an inner conductor and a coaxial outer conductive housing.
There are, of course, many protective devices employed on the typical electric utility transmission and distribution system. However, the last line of defense to protect a generator against overload damage is a circuit breaker in the isolated phase bus duct run which isolates the generator in case of a short circuit or fault in the step-up transformer. Due to the high energy flow which must be interrupted during fault conditions, circuit breakers often require extensive maintenance following such interruption operations.
To facilitate this maintenance, disconnect switches are typically provided on either side of the circuit breaker to isolate the breaker from any source of high potential. The disconnect switches are not required to interrupt normal load current but may be called upon to interrupt the rather sizable magnetizing current of the transformer.
One of the primary requirements of a disconnect switch for use in isolated phase bus duct is that it exhibit low losses under normal circuit conditions. This is desirable not only to avoid unnecessary waste of valuable electrical energy but to reduce heating caused by current flow through high resistance connections. Such heat can be extremely destructive to the switch, the bus, and associated apparatus. In addition, the switch must provide reliable operation when called upon even after long periods of inactivity, and must have the ability to withstand the extremely high electrodynamic forces produced by high fault current conditions. A switch must also provide convenient means for inspection and maintenance while at the same time maintaining the electrical integrity of the switch during normal operating conditions.
Prior art disconnect switches have been used in isolated phase bus applications. It would be desirable, however, to provide a switch having a higher degree of reliability and withstand capability, while at the same time reducing the cost of the switch.